In the cited patents, a hydrostatic transmission is disclosed as including a hydraulic pump unit and a hydraulic motor unit positioned in opposed, axially aligned relation on opposite sides of a wedge-shaped swashplate. The pump unit is connected to an input shaft driven by a prime mover, while the motor unit is ground to the stationary transmission housing. An output shaft, coaxial with the input shaft and drivingly coupled to a load, is connected to the swashplate in torque-coupled relation. When the pump unit is driven by the prime mover, hydraulic fluid is pumped back and forth between the pump and motor unit through ports in the swashplate. As a result, three torque components, all acting in the same direction, are exerted on the swashplate to produce output torque on the output shaft for driving the load. Two of these torque components are a mechanical component exerted on the swashplate by the rotating pump unit and a hydromechanical component exerted on the swashplate by the motor unit. The third component is a purely hydrostatic component resulting from the differential forces created by the hydraulic pressures acting on circumferentially opposed end surfaces of the swashplate ports, which are of different surface areas due to the wedge shape of the swashplate.
The torque-coupled connection of the wedge-shaped swashplate to the output shaft is such that the angular orientation of the swashplate relative to the axis of the output shaft may be varied. When the input face of the swashplate, juxtaposed with the pump unit, is perpendicular to the output shaft axis, the transmission ratio, i.e., speed ratio, is set to 0:1 (neutral). On the other hand, when the output face of the swashplate, to juxtaposed with the motor unit and angularly offset from the input face, is perpendicular to the output shaft axis, the transmission is set to 1:1. Since the swashplate may be pivoted (stroked) to any angular orientation between the 1:0 and 1:1 ratio positions to set any intermediate ratio, the speed ratio of the transmission is continuously (infinitely) variable.
In the hydrostatic transmissions disclosed in the referenced patents, various embodiments of controllers are disclosed for stroking a wedge-shaped swashplate to vary transmission ratio. These ratio controller embodiments utilize either a single, double-acting piston or a pair of opposed pistons driven in opposite axial directions by pressurized hydraulic fluid tapped from the swashplate ports for exerting stroking forces (moments) on the swashplate to vary the swashplate angular orientation and thus to increase (up-stroke) or decrease (down-stroke) transmission ratio. The stroking force exerted by these actuating pistons is solely the product of hydraulic fluid pressure and the area of the piston face exposed to the hydraulic fluid pressure. Unfortunately, the force(s) generated by the piston(s) to set a transmission ratio and the stroking force necessary to change transmission ratio vary according to the angular orientation of the swashplate. For example, at approximately the 0.5:1 ratio position, when the angles of the input and output faces of the swashplate relative to the output shaft axis are equal, the opposing forces of the pump hydraulic fluid acting on the swashplate faces are essentially equal. Thus the requisite force(s) exerted on the swashplate by the activating piston(s) to set the 0.5:1 ratio position of the swashplate is minimal. However, the forces generated by the activating piston(s) to up-stroke or down-stroke the swashplate from the 0.5:1 ratio position must increase disproportionately to the increase in hydraulic fluid pressure available from the swashplate ports. Consequently, the actuating piston(s) must have sufficiently large face surface areas in order to generate requisite forces to both vary and set transmission ratios over the available range of ratios, which may include a reverse range beyond the 0:1 ratio position and a limited overdrive range beyond the 1:1 ratio position. A large actuating piston(s) adds size and weight to the transmission.
Another characteristic of the ratio controllers disclosed in the referenced patents is that the swashplate is mounted on a transverse pivot pin fixed to the output shaft to establish a swashplate pivot axis intersecting the output shaft axis at a right angle. Axially directed stroking forces are then exerted on the swashplate at locations radially offset from the output shaft axis to generate moments necessary to vary and set transmission ratios. Unfortunately, these moments exert bending forces on the output shaft, which place high loadings on bearings and support structure and can cause output shaft deflection. Consequently, the output shaft and its supporting components must be sized accordingly to withstand such bending moments. This further adds to the size and weight, as well as cost, of the transmission.